1. Field of the Invention
This invention relates to new lacquer polyisocyanates which have at least partially blocked isocyanate groups and which allow the production of nonyellowing lacquers due to incorporated stabilizers, to a process for the production of the new lacquer polyisocyanates and to the use thereof as isocyanate component in thermosetting two-component polyurethane lacquers.
2. Description of the Prior Art
Irrespective of whether polyurethane stoving lacquers are processed as a solution or as a powder, they consist essentially of a hydroxyl component and a polyisocyanate component, the NCO groups of which are completely or partially blocked by a blocking agent to prevent them from reacting prematurely. This second possibility is described, for example, in U.S. Pat. No. 4,055,550 or in German Offenlegungsschrift No. 2,842,641. A cross-linking reaction forming the lacquer only occurs under the influence of heat (for example, heating a coated substrate to 180.degree. C. for 30 minutes) with release of the blocking agent.
However, the known thermosetting two-component polyurethane lacquers exhibit a more or less marked tendency to yellow under the influence of the high temperatures which are necessary for stoving. This fact is particularly disadvantageous in continuous industrial lacquering when, for example, stoving is either carried out for too long or at too high a temperature due to technical disturbances.
In such a case, a nonuniform series of different colored parts are produced. The production of a nonyellowing, clear lacquer on, for example, a silvery background is also a problem because the disturbing effect is particularly conspicuous due to the lacking pigment.
Attempts have been made to overcome this disadvantage with the development of polyisocyanates which are particularly color-stable to heat. Such polyisocyanates described, for example, in published European Patent Application No. 798 and in German Offenlegungsschrift No. 2,842,641, are based on trimerized 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone-diisocyanate or IPDI). A particular disadvantage of the lacquer polyisocyanates or lacquering processes described in the prior publications mentioned is the restriction to only one of the commercially, easily accessible polyisocyanates, quite apart from the fact that the yellowing resistance of the lacquers produced using this specific lacquer polyisocyanate, in particular, clear lacquers, is in need of improvement.
German Offenlegungsschrift No. 1,720,711 does indeed indicate a course of action as to how various polyisocyanates may be modified such that they allow for the production of lacquers having an increased yellowing resistance. However, the modified polyisocyanates which are recommended in this publication suffer from considerable disadvantages. Thus, the production of the stabilized polyisocyanates according to the publications mentioned is commercially expensive, because it is necessary to react the N,N-dialkylhydrazine compound used for stabilization with a large excess of low molecular weight polyisocyanate and then to remove the unreacted excess polyisocyanate by distillation. The use of N,N-dialkyl-substituted hydrazines as a stabilizer is also a problem. Lacquer films produced using such polyisocyanates always still contain basic N-atoms which in turn are readily open to an attack by atmospheric acids, thereby reducing the resistance of the lacquer, particularly in an industrial climate.
An object of the present invention is to provide new lacquer polyisocyanates which may be produced in an easily controllable reaction by modifying any polyisocyanates, which allow for the production of nonyellowing stoving lacquers. This object may be achieved by the process according to the present invention which is described in more detail in the following.